1. Field of the Invention
The present invention relates to a CDMA mobile communication system and more particularly, an apparatus and method for searching PN offsets.
2. Background of the Related Art
Generally, a Code-Division Multiple Access (CDMA) mobile communication system is made up of at least a mobile station and a base station. The mobile communication system signal, which is modulated and transmitted from the mobile station, is received and then demodulated by the base station, and restored to original signals. At this time, the mobile communication signal includes a pilot signal for time synchronization and an electric power control. The pilot signal is spread with a PN (pseudo noise) code, which is specific to each user's mobile terminal, and is transmitted to the base station. The base station can demodulate the received signals from particular PN offsets, which are time delay offsets of multiple path components and can be found by a search operation.
It is possible to search for the PN offsets by executing the despreading of every possible offset. After the despreading with every possible offset, the energies for each offset are estimated. The demodulation of the received signal is performed from the PN offsets having a bigger energy value than a specific threshold value among a plurality of energy values. The device that performs the above-described operations is generally known as “searcher”.
FIG. 1 is a block diagram showing a related pilot signal searcher. As shown in FIG. 1, a searcher 100 includes a PN (pseudo noise) code generator 1, a despreading device 2, a coherent accumulator 3, an energy calculator 5, a non-coherent accumulator 6, and a sorter 8. An input signal received through an antenna (not shown) is input into the despreading device 2 after being separated into an I (in-phase) component and a Q (quadrature) component. When the I and Q component input signals are both input into the despreading device 2, the PN code generator 1 creates corresponding I and Q PN codes for input into the despreading device 2. The PN code generator 1 generates the PN codes corresponding to one PN offset. The I and Q component input signals are input into the despreading device 2 as many as the number determined at the coherent accumulator 3 and the non-coherent accumulator 6. The despreading device 2 despreads the received input signals by using the inputted PN codes and transmits output signals into the coherent accumulator 3. Output signals from the despreading device 2 are the I and Q components. That is to say, the despreading device 2 despreads the first input signal by using relevant first PN code among the PN codes, and then despreads the second input signals that are input after the first input signal using the next relevant PN code. The above-described operations are repeated according to the number of input signals determined by the coherent accumulator 3 and the non-coherent accumulator 6.
The coherent accumulator 3 accumulates each inputted component (I and Q). The accumulated I and Q components yield an energy value by squaring each component and adding both squared results with an energy calculator 5. To estimate the energy value more accurately, the energy value from energy calculator 5 is averaged for a given period of time by the non-coherent accumulator 6. Here, the energy value means an energy value for one PN offset.
To estimate the energy for different PN offsets, the same process described above is again performed except the PN code changes, which is generated with different PN offset by the PN generator 1. The non-coherent accumulator 6 determines averaged energy values for a plurality of energy values, which are the energy values for possible PN offsets. The process is expressed in the following numerical formula, where the energy value of one offset is:
  E  =            ∑              n        =        1            N        ⁢                  {                                                            (                                                      ∑                                          m                      =                      1                                        M                                    ⁢                                                            I                      ⁡                                              (                                                  nM                          +                          m                                                )                                                              ·                                                                  PN                        i                                            ⁡                                              (                                                  nM                          +                          m                                                )                                                                                            )                            2                                      I              ⁢                                                          ⁢              coherent              ⁢                                                          ⁢              accumulation                                +                                                    (                                                      ∑                                          m                      =                      1                                        M                                    ⁢                                                            Q                      ⁡                                              (                                                  nM                          +                          m                                                )                                                              ·                                                                  PN                        q                                            ⁡                                              (                                                  nM                          +                          m                                                )                                                                                            )                            2                                      Q              ⁢                                                          ⁢              coherent              ⁢                                                          ⁢              accumulation                                      }            /      N      where I(.) and Q(.) are the input signal of I and Q components, respectively and PNi and PNq mean the PN code for I and Q, respectively. The numerical formula exemplifies M times of coherent accumulation and N times of non-coherent accumulation.
The sorter 8 sorts the multiple averaged energy values for every possible PN offset from the non-coherent accumulator 6 in a descending order. In this way, if sorting is performed based on an energy mean value yielded for every possible PN offset, a finger manager (not shown) allocates PN offsets, which have bigger energy mean value than a specific energy mean value threshold, to fingers. Then, the fingers demodulate the received signal from the assigned PN offsets.
FIG. 2 is a diagram that illustrates a coherent calculation for one PN offset is 32 PN chip and a non-coherent calculation is 4. As shown in FIG. 2, 32 pieces of input signals for the first 32 PN codes are respectively despreaded, their outputted values are accumulated, then an energy value is determined, and the coherent calculation that outputs one energy value can be performed again. For the next 32 PN codes, the next 32 pieces of input signals are respectively despreaded and the coherent calculation, which outputs one energy value, is again performed. Each energy value is calculated by repeating the same process (e.g., as shown in FIG. 2, twice more) and then, through the non-coherent calculation, which calculates the mean value of these energy values (e.g., 4), an energy value for one PN offset can be calculated.
However, as described above, the related art CDMA mobile communication system has various disadvantages. In the CDMA mobile communication system, the searcher of a base station should search for each time delay for every multiple paths and the signals of many user's mobile terminals. With the related art searcher, there has been a limit to searching for a pilot signal in a high speed. Thus, there exists a need to reduce time to search for PN offsets.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.